Voltammetric Determination of Nitrate Ion at Parts-per-Billion Levels Mario E. Bodini and Donald T. Sawyer* Department of Chemistry, University of California, Riverside, Calif. 92502
A new voltammetrlc method for the determlnation of nitrate ion at a pyrolytic graphlte electrode has been developed. It is based on the electrocatalytic reductlon of nitrate Ion which occurs at -1.0 V vs. SCE when copper metal and cadmlum metal have been electrochemically deposited onto the surface of the working electrode. The presence of iron, chlorlde, and sulfate ions, and of organic matter, does not interfere with the analysls, but nitrite ion is reduced at the same conditions. With the sample solution adjusted to contain 0.1 M NaH2P04,10 pM CdC12, and 50 pM CuC12, linear Calibration curves are obtained for the concentration range from 1 pM (62 ppb) to 1 mM (62 ppm) nitrate ion. For higher concentrationsof NO3- (up to 10 y), a linear calibration curve is obtained by use of solutions that contain 0.1 M KCI, 0.01 M HCI, 0.2 mM CdC12,and 1.0 mM in CuCI1. The method has been applied to samples of lrrlgation water and to water extracts of airborne particulate samples.
Numerous analytical methods have been developed for the determination of nitrate. Those based on spectrophotometry are the most common, and are based either on the oxidation or on the nitration of an organic compound (1-7). However, these methods require the use of toxic reagents, are subject to numerous interferences, often have a nonlinear response, and provide a lower detection limit of about 5 ppm of nitrate. Several electrochemical methods for the determination of nitrate ion also have been proposed. The polarographic methods are based on the catalytic waves that result from the presence of V022+ (8),La3+ ( 9 ) ,Ce3+ ( 9 ) ,molybdate ion (IO), Zr02+ ( I I ) , and chromium(II1) complexes of glycine (12).The suppression of such catalytic activity by FeS04 also has been used for the determination of N03- (23).Another approach has been to measure the polarographic reduction of 4-nitro2,6-xylenol ( 1 4 ) , which is formed by reaction of nitrate ion with 2,6-xylenol. A direct electrochemical method recently has been proposed through the use of a rotating cadmium disk electrode ( 1 5 ) .However, the calibration curve is linear only for a narrow range of concentrations. The same problem of nonlinear response to nitrate concentration is common to all of the polarographic methods. An improved method for the determination of trace quantities of nitrate ion by linear scan voltammetry at a stationary mercury drop electrode has been reported recently (16).An anion-exchange membrane is used to concentrate the nitrate ion, but this approach has the disadvantage that it also concentrates anionic interferences, especially sulfate ion. Such anionic interferences are the main disadvantage of the potentiometric determination of nitrate ion by use of ion-specific electrodes. Another general method for the determination of nitrate ion is based on its reduction to nitrite ion and the colorimetric determination of the latter. Although several procedures have been described to accomplish a quantitative reduction to NOz-, including the use of an enzyme reaction (17),the most common approach is to pass the solution through a column that contains a Cu-Cd catalyst (18).The major problem is the maintenance of an active Cu-Cd surface. One way to avoid this problem is to generate the active
surface in a reproducible way every time an analysis is to be done. This led us to consider the electrochemical deposition of these metals onto the surface of a solid electrode as a way to achieve an active, reproducible reduction catalyst. The reaction between nitrate ion and the catalyst on the electrode surface in turn produces a catalytic current that can be used for the determination of nitrate ion in the solution. The present paper describes the behavior of these systems and the optimum conditions for the trace determination of nitrate ion by linear sweep voltammetry.
EXPERIMENTAL Cyclic voltammetry was performed by use of a three-electrode potentiostat that was constructed from solid-state operational amplifiers (19) and a Hewlett-Packard Model 7030A X-Y recorder. The electrochemical cell consisted of a 100-ml electrolytic beaker and a Leeds and Northrup polyethylene electrochemical cell top. The cell top had provision for inserting the graphite working electrode, the auxiliary compartment (a Pyrex tube with a fine porosity frit on the end), the Luggin capillary which contained the reference electrode, a bubbler to deaerate the solutions with argon, and a short piece of glass tubing to flow argon above the solution surface while the cyclic voltammograms were recorded. The auxiliary electrode was a small piece of platinum mesh. The reference electrode consisted of a silver wire coated with AgCl in a Pyrex tube closed with a small soft glass cracked-bead tip. The electrode was filled with a solution of tetramethylammonium chloride with the concentration adjusted such that the electrode potential was 0.00 V vs. SCE. The pyrolytic graphite electrodes were prepared from a piece of 5-mm diameter pyrolytic graphite rod (Catalog No. EPC-1) from the General Electric Corp., Detroit, Mich., which was fitted into a piece of glass tubing and sealed with epoxy resin. The surface of the electrode was polished by use of decreasing grades of carborundum paper, and finally with 5-, 2-, 1-, and 0.3-pm alumina powder. All of the solutions were prepared with doubly-distilled water and ACS Reagent Grade chemicals. The standard solutions of NaN03 and KNOl were prepared determinately from the salts after they were dried for 1 h a t 110 "C. The stock solutions of CdC12 and CuClz were standardized by titration with EDTA (20). All measurements were made a t 25 "C.
RESULTS AND DISCUSSION The present voltammetric method is based on the catalytic reduction of nitrate ion which occurs in the presence of freshly deposited copper and cadmium on an inert solid electrode. Figure 1illustrates the cyclic voltammograms for a blank solution that contains 0.1 M NaH2P04,lO yM CdClZ, and 50 MM CuC12 (curve A) and for a 0.1 mM Nos- sample (curve B) (1 HMNO3- = 62 ppb NOS- = 14 ppb N, 1mM Nos- = 62 ppm NO3- = 14 ppm N). The difference in peak current a t -1.0 V is directly proportional to the concentration of nitrate ion. Analytical Procedure. For the analysis of samples that contain nitrate ion in the concentration range between 1yM and 1mM, the solution needs to contain 0.1 M NaH2P04 (pH 4.5), 10 pM CdClZ, and 50 pM CuC12. Under these conditions the calibration curve is linear (curves A and B, Figure 2). For concentrations of nitrate ion higher than 1mM some deviation from linear response occurs (curve C, Figure 2) but an alternative procedure in which the solution contains 0.1 M KC1, 0.01 M HC1 (pH Z), 0.2 m M CdC12, and 1mM CuClz yields a calibration curve with linear response up to 10 mM nitrate ion (curve D, Figure 2). The electrocatalytic current is determined by negatively ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
485
I-
z W
LL LL
3
0
0-
1 0
1
I -08
-04
I -1 2
E, V v s SCE
Flgure 1. Comparison of the cyclic voltammograms at a scan rate of 100 mV/s for a blank solution containing 0.1 M NaH2P04,10 pM CdC12, and 50 yM CuClp (curve A) and for the same solution with the addition of 0.1 mM NaN03 (curve B)
2 1000
800
4
a
600
6
400
.-
4
6
1
0
A
Fc
cNOi,
200
mM
1
0
8
2
4
I 6
I
8
1
0
Q
d
._
40 20
a
I 6f
200
400
20
40
A
3.
A 600
800
1000
60
80
100
.-a
Figure 2. Calibration curves for the peak current for the electrocatalytic reduction of nitrate ion as a function of concentration. Curves A, B, and C for solutions that contain 0.1 M NaH2P04,10 pM CdC12, and 50 pM CuCI2.Curve D for a solution that contains 0.1 M KCI, 0.01 M HCI, 0.2 mM CdClp, and 1.O mM CuCIp
sweeping the voltage a t a scan rate of 100 mV/s until the catalytic reduction peak is observed. At this point the scan is reversed in the positive direction to +0.2 V vs. SCE in the case of solutions that contain 0.1 M NaH2P04, and to +0.4 V vs. SCE for solutions that contain 0.1 M KC1 and 0.01 M HC1. The reverse positive scan is necessary to reoxidize the metals and remove them from the surface of the electrode. For the analysis of samples with high concentrations of nitrate ion the electrode should be anodized for 30 s at f0.4 V between runs to ensure that its surface is clean and to obtain reproducible results. 486
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
I '
I 0
I -0 4
I
I
-0 8
-1
2
E, V vs SCE Flgure 3. Cyclic voltammograms at a scan rate of 100 mV/s for 0.1 M NaH2P04solutions that contain: (a) 50 pM NaN03 (curve A), (b) 50 y M NaN03 and 10 y M CdCI2(curve B), (c) 50 yM NaN03 and 50 yM CuClp (curve C), and (d) 50 y M NaN03, 10 pM CdCI2, and 50 yM CuCb (curve D)
For the analysis of'samples with low concentrations of nitrate ion (1 yM to l mM), the peak currents are measured relative to the limiting current for the second reduction of copper ion (-0.4 V, curve D, Figure 3). In this way copper is used as a pilot ion to set the current axis of the recorder; for maximum sensitivity and accurate current measurements, the zero current axis is off-scale of the recorder. This is not necessary for samples with higher concentrations of nitrate ion, and the peak current can be measured directly from the zero current axis. The method of standard addition to the samples has proved especially convenient to check for interferences and to verify the calibration curve, in particular when the samples are complex and a blank solution is not available. In these cases two or three standard additions permit corrections to be made for the background and provide a check of the linearity of the response. T o verify that the system is behaving properly, the cyclic voltammograms are repeated three times to check their reproducibility. The solutions are stirred for 30 s and then allowed to stand for another 30 s between runs. Selection of Analytical Conditions. a) Electrode. Several solid electrodes have been studied in 0.1 M KC1 to determine the best analytical conditions. Glassy carbon gives a satisfactory response for nitrate ion in the presence of copper and cadmium, but exhibits a large residual current which decreases the analytical sensitivity of the determination. The same problem occurs with a gold electrode, which yields a reduction peak for nitrate ion in the presence of cadmium ion at -1.05 V vs. SCE. An interesting characteristic of this electrode is that it does not require the presence of copper ion in the solution, and the peak current for nitrate ion has a linear dependence with the concentration of nitrate ion up to 5 mM. Another disadvantage of the gold electrode is that it requires pre-tathodization a t -1.0 V vs. SCE to obtain an optimum response. A copper electrode also has been tried, but its surface apparently changes with time to give nonreproducible results. Pyrolytic graphite appears to be the best electrode material because of its extremely low residual current (about 1PA a t -1.0 V vs. SCE). Pretreatment of the electrode is not required to obtain a good response and reproducible results. b) Supporting Electrolyte. Several supporting electrolytes have been evaluated by use of a pyrolytic graphite
I
I
I
-
Table I. Effect of the Scan Rate upon the Electrocatalytic Reduction of 0.1 mM Nitrate Ion in the Presence of 0.1 M NaHZP04,50 pM CuC12, and 10 pM CdClz Scan rate, mV/S 500 200 100 50 20 10 5
ip,
pA
22 16.5 12.5 9.2 6.2
4.5 3.7
ibkgd (-0 4 V),
11 5 3
FA
ip/ibkgd
1.0
2.0 3.3 4.2 4.2 5.2 4.5
1.0
3.7
2.2 1.2
t-
z w
K K 3 0
0-
0
I
I
-04
-08
I -1
2
E, V v s SCE
working electrode. Three systems (KC1-HC1 (pH 2.0), NaH2P04 (pH 4.5),and NH4Cl-NH3 (pH 8.5)) have proved useful and provide a range of solution conditions for various types of nitrate samples. The concentrations of CdC12 and CuClz that have been used in these supporting electrolytes have been kept at minimum levels to reduce the background current. 0.1 M KC1-0.01 M HC1 System. With this supporting electrolyte, a catalytic peak due to the presence of nitrate ion is observed a t -0.85 V vs. SCE in the presence of both CuC12 and CdC12. A study of the effect of the concentration ratio of CuCl,-to-CdClz indicates that a 1:l ratio is the most favorable for low concentrations of nitrate ion, and that a 5:l ratio is the best for high concentrations. However, for low nitrate concentrations in the presence of 20 pM CuCl2 and 20 pM CdC12, the response is linear only from 10 pM to 1.0 mM NOB-. Below 10 pM NOa- a random deviation from linearity is found, whereas for concentrations higher than 1 mM NO3- the response becomes nonlinear with an increasingly negative deviation. When the sample solution contains l mM CuC12 and 0.2 mM CdClz the response is linear up to 10 mM N03-. However, the large background current due to the higher concentrations of CuC12 and CdCl2 precludes the use of these conditions for the determination of low concentrations of nitrate ion. 0.1 M NHdCl-0.01 M NH3 System. With this supporting electrolyte, nitrate ion yields an electrocatalytic wave when the solution also contains CuC12 and CdCl2 in a 1:lmole ratio. An excess of CuClz does not increase the catalytic peak, but instead enhances a second reduction peak that occurs a t a more negative potential. This second reduction apparently is due to the reduction of nitrite ion. At low concentrations of CuC12 and CdClz (each 10 pM), this electrolyte system yields a linear response for nitrate ion concentrations between 10 pM and 100 pM. At higher concentrations of the metal ions (0.2 mM CuClz and 0.2 M CdClZ), a linear response is obtained between 0.5 mM and 7.0 mM Nos-. Below 0.5 mM N03-, the background current is too large to allow accurate determinations. 0.1 M NaHzP04 System. This system, which has a solution acidity of p H 4.5, provides the best characteristics for nitrate ion determinations in terms of linear calibration curves and sensitivity (curves A and B, Figure 2). Figure 3 illustrates cyclic voltammograms for 50 pM Nos- in the absdice of CuC12 and CdCl2, in the presence of CdC12 only, in the presence of CuClz only, and in the presence of CuC12 and CdC12 at a mole ratio of 51. In the absence of CuC12, the reduction of cadmium ion is hardly detectable and there is not any catalytic reduction of NOS-. Conversely, in the absence of CdClz the two reduction steps for CuClz are present, but catalytic reduction of Nos- is not observed. Instead, the reduction of hydrogen ion and solvent occurs at about -0.9 V vs. SCE. A distinct catalytic reduction peak develops at -1.0 V vs. SCE when the solution contains both Cu2+ and Cd2+: these data establish
Figure 4. Voltammograms at a scan rate of 100 mV/s for solutions that contain 0.1 M NaH2P04, 10 pM CdC12, and 40 pM NaN03 plus the indicated%oncentrations of CuC12
I
I
/
k
z W
LL
LL 3 0 0-
0
I -0 4
E, V
I -0 8
I -I
2
v s SCE
Figure 5. Voltammograms at a scan rate of 10 mV/s for solutions that contain 0.1 M NaH2P04,10 pM CdCI2, 50 pM CuCI2,and 0.1 mM NaN03 plus the indicated concentrations of KN02
that the simultaneous presence of the two metal ions is essential. The effect of the concentration ratio of CuC12 to CdC12 in 0.1 M NaH2P04 upon the electrocatalytic NO3- reduction peak is illustrated in Figure 4.An excess of CuClz enhances the catalytic peak up to a 5:l CuClZ-CdClz mole ratio. In contrast, an excess of CdClz ion does not provide an enhancement or analytical advantage. c ) Scan Rate. A voltage scan-rate of 100 mV/s offers the optimal response sensitivity for the determination of nitrate ion. Table I summarizes the peak current and its ratio to the background current as a function of scan rate for a 0.1 mM NO3- solution. Although the signal-to-background ratio is somewhat larger a t scan rates slower than 100 mV/s, the coincident decrease in peak current more than offsets this apparent advantage. At scan rates faster than 100 mV/s, the background current increases more than does the peak current for nitrate ion. The technique of differential pulse voltammetry has been tried in an attempt to improve the sensitivity of the method. Unfortunately, the fastest scan rate available with the instrument that was used (Princeton Applied Research Model 174) is only 10 mV/s. The loss in sensitivity that results from such a slow scan rate more that offsets the improved signal-to-background current ratio that results from the differential pulse experiment. ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
487
Table 11. Determination of Nitrate Ion in Samples of Irrigation Water
Table 111. Determination of Nitrate Ion in WaterExtracts of Airborne Particulate Samples
Nitrate-nitrogen Sample Bradley Philbrick No. 3 Hackett Blosser Telephone Simes Philbrick No. 4
CNOn-,
Electrocatalytic, ppm Technicon, ppm 12.1 4.1 1.3 9.9 13.6 10.6 3.5
10.4 3.4 0.7
8.5 13.1 11.4 3.3
Sample
Electrocatalytic
Technicon
1
2 3 4
4.75 4.64 3.98 3.4
5
2.7
6
3.37
4.57 5.82 3.39 3.03 3.06 3.03 2.55 3.33 3.12
7 8
9 Interferences. Among the metal ions that are likely to be present in water samples (21) and interfere with the determination of nitrate ion, iron appears to be the most important. In the presence of NaHZP04, a white precipitate is formed when the solution is made 1.4 mM in Fe3+ (80 ppm). However, the measurements in the presence of this precipitate are not affected and the calibration curve is linear for NO3- concentrations between 1W Mand 0.5 mM. For higher concentrations of nitrate ion, some deviation from linearity is observed. For such samples the KCl-HC1 supporting electrolyte system can be used to eliminate the problem of interference by iron ions. Among the anions that occur in significant amounts in water samples (21),sulfate and chloride ions do not affect the analytical determination of nitrate ion. For high salt concentrations, the slope of the linear calibration curve decreases because of the increase in the viscosity of the solutions. The presence of nitrite ion is a direct interference because it is reduced at the same potential as nitrate ion. Cyclic voltammograms at slower scan rates (10 mV/s) for N03- and for combinations of NO3- and NOz- exhibit two reduction peaks (Figure 5 ) . The first a t -0.95 Vis due to N03- and the second at -1.1 V is due to NOz-. The curves of Figure 5 also indicate that the first reduction step for CuClz (Cu(I1) Cu(1)) is catalytic for the reduction of nitrite ion and that an additional reduction process occurs a t -0.55 V vs. SCE. Calibration curves for nitrite ion under these conditions are nonlinear for the CuC12 catalytic reduction, for the reduction peak a t -0.55 V, and for the peak at -1.1 V. However, the responses appear to be reproducible and may have some analytical utility. The presence of significant amounts of organic matter in the samples alters the slope of the calibration curves, although the linearity of response is maintained. For such systems the method of standard addition provides a convenient means to correct for the interfering effect. Applications. The electrocatalytic method has been applied to the determination of nitrate ion in samples of irrigation water and in samples from water extracts of airborne particulates. For the samples of irrigation water, a 25-ml aliquot was combined with 5 ml of 1M NaHZP04,50 pl of 10 mM CdC12, and 250 ~1 of 10 mM CuC12, and diluted to 50 ml with double-distilled water. These solutions then were degassed for 5 min with argon before the cyclic voltammograms were recorded by the procedure outlined at the beginning of this section. The measurement was repeated three times to ensure that the system was giving a reproducible response. The method of standard addition also was applied to check the calibration curve. Table I1 summarizes the results that have been obtained for these samples and compares them with those obtained by the Technicon colorimetric method. The latter is based on the reduction of NOS- by an active Cu-Cd catalyst to NOz-, followed by reaction with an azo dye (18). For the water-extract samples of airborne particulates, the
-
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
mM
2.59
3.67 3.47
preparation of the analytical solutions was the same as for the samples of irrigation water, except that a 2.5-ml aliquot of sample was used. For these samples, some interference resulted from the organic matter that was present. However, by use of the method of standard addition, reproducible quantitative results have been obtained. Such an approach accommodates any change in slope of the calibration curve as well as any change in the background current. The analytical results are summarized in Table I11 and are compared with those obtained by the Technicon colorimetric method. In both comparisons the electrocatalytic method yields higher results for most of the samples. This may be due to side reactions with organic matter by the NOz- that is produced for the colorimetric reaction of the Technicon method. CONCLUSIONS The present method is based on the reduction of nitrate ion at a Cu-Cd catalyst that is formed on the surface of a pyrolytic graphite electrode. By use of a fast scan rate and an excess of CuC12, the reduction of NOS- and of NOz- is indistinguishable and the measured peak current corresponds to the total reduction process (probably to N20 or Nz). Some interaction between copper(1) and nitrite ion appears to occur because the current associated with the reduction of copper(I1) increases with the concentration of nitrite ion. This effect is not observed with the reduction of copper(1). The electrocatalytic method for the determination of nitrate ion is comparable in its utility and applicability with both the Technicon colorimetric and the ion-selective electrode methods. Furthermore, it is applicable to a substantially wider range of nitrate ion concentrations in terms of providing a quantitative linear response. The method also has higher sensitivity and is less affected by interferences than the other techniques. Because the measurement procedure utilizes cyclic voltammetry, the response curve indicates the presence of interferences and provides the means to correct for them. ACKNOWLEDGMENT We are grateful to Mark Morrison of this department for assistance in the preparation of the electrodes, to Parker Pratt (Department of Soil Sciences, U.C.R.), Thomas Mischke (this department), and Jerry Smith (this department), for supplying samples and the comparison analyses, and to the Catholic University of Chile for a Predoctoral Fellowship to M.E.B. LITERATURE C I T E D (1) I. M. Kolthoff and G. E. hloponen, J. Am. Chem. SOC., 55, 1448 (1933). (2) H. Riehrn, Fresenius’ Z.Anal. Chem., 81, 439 (1930). (3) M. J. Taras, Anal. Chem., 22, 1020 (1950).
A. M. Hartley and R. i. Asai, Anal. Chem., 3 5 , 1214 (1963). G. B. Jones and R . E. Underdown, Anal. Chem., 25, 806 (1953). A. D. Westiand and R. R. Langford, Anal. Chem., 28, 1996 (1956). "Standard Methods for the Examination of Water and Wastewater", 12th ed., American Public Health Association, inc., New York, N.Y., 1965, pp
195-205. I. M.
Koithoff,W. E. Harris, and G. Matsuyama, J. Am. Chem. SOC.,66, 1782
(1944).
J. W. Coliat and J. J. Lingane, J. Am. Chem. Soc., 76, 4214 (1954). M. G. Johnson and R. J. Robinson, Anal. Chem., 24,366 (1952). Ph. Mecheiynck and C. Mechelynck-David, Anal. Chim. Acta, 21, 432
(1959).
R. E. Hamm and C. D. Withrow, Anal. Chem, 27, 1913 (1955). M. C. Rand and H. Heukeiekian,Anal. Chem., 25, 878 (1953). A. M. Hartiey and D. J. Curran, Anal. Chem., 3 5 , 686 (1963). R . J. Davenport and D. C. Johnson, Anal. Chem., 45, 1979 (1973).
(16) G. L. Lundquist, G. Washinger, and J. A. Cox, Anal. Chem., 47, 319 (1975). (17) D. R. Senn, P. W. Carr, and L. N. Klatt, Anal. Chem., 48, 954 (1976). (18) E. D. Wood, F. A. J. Armstrong, and F. A. Richards, J. Mar. Bid. Assoc. U.K., 47, 23 (1967). (19) A. D. GooisbyandD. T. Sawyer, Anal. Chem., 39,411 (1967). (20) F. J. Welcher, "The Analytical Uses of EthylenediaminetetraaceticAcid", D. Van Nostrand Co., New York, N.Y., 1960. (21) J. W. Clark, W. Viessman, Jr., and M. J. Hammer, "Water Supply and
Pollution Control", 2d ed., international Textbook Co., London, England, 1971, p 234.
RECEIVEDfor review September 27,1976. Accepted December 6,1976. This work was supported by the National Science Foundation under Grant No. CHE 73-05204.
Normal Pulse Voltammetry in Electrochemically Poised Systems J.
Lee Morris, Jr., and Larry R. Faulkner"
Department of Chemistry, University of Illinois, Urbana, Ill. 6 180 1
Normal pulse polarography generally gives a severely distorted view of solutlon composition in a poised system, because the working electrode is active during the interval between pulses. Two ways for preventingthe distortion were explored. In one scheme, the conventional depolarized reference electrode was replaced by a quasi-referenceelectrode (QRE), and the usual potential waveform was modifled so that the potential was held at 0 V vs. ORE between pulses. This method permits analysis In poised systems without distortion. The second scheme Involved the insertion of an electronic switch to free the working electrode from potentiostatlc control except during potential pulses. Restoration proceeds by dlffusion, but is Inhibited by coulostatic discharge of the double layer.
The past several years have seen widespread exploitation of the decreased analysis times and increased signal-tobackground ratios offered by normal pulse polarography ( 1 , 2 ) , as compared to the conventional dc method. However, there is a generally unrecognized drawback to this pulse technique that severely affects its application to electronically poised systems, Le., systems which display a true equilibrium potential because both oxidized and reduced forms of a given redox couple are present. For the purpose of this discussion, we include among poised systems not only those fulfilling the strict sense of the term, but all others for which the voltammetric curves do not show a readily predictable region of zero shows faradaic current. For example, Fe(II1) in 1 M "03 zero current at the dropping mercury electrode only when the reduction of Fe(1II) is exactly compensated by the oxidation of Hg. The difficulty in these cases stems from the form of the potential program applied to the working electrode. That program involves the imposition of a base potential during the long waiting periods (0.5-4 s) between the potential pulses which stimulate the current that is actually sampled. The usual assumption is that negligible electrolysis occurs during the waiting period, so that the solution composition, as sampled in the pulse, is the same as that of the bulk. In a poised system, this assumption can hold only if the base potential coincides fortuitously with the electrode equilibrium potential. Otherwise, electrolysis during the waiting period modifies the solution composition near the electrode, and the pulse measurement gives a distorted view of the bulk composition. This problem was brought to our attention during an ex-
ploration of pulse methods as means for enhancing the sensitivity of the ferrioxalate actinometer ( 3 ) .The analysis step of interest is the measurement of a small concentration of Fe(I1) in the presence of a high concentration of Fe(II1). By itself, Fe(I1) can be measured via normal pulse polarography at a solid electrode carried through a positive-going scan. The base potential would be near 0.2 V vs. SCE. In the presence of Fe(III), this base value causes electroreduction which yields Fe(I1) directly and destroys the ability of the experiment to reflect the true bulk concentration of Fe(11). This particular analysis problem was considered recently by Parry and Anderson ( 4 ) ,who were able to apply pulse polarography to it by finding a supporting electrolyte (0.1 M sodium pyrophosphate) in which the Fe(II)/Fe(III) couple falls within the working range of the DME and also is sufficiently irreversible that a well-defined zero-current region divides the anodic and cathodic waves. They could determine Fe(1I) and Fe(II1) separately by setting the base potential in the zero-current region and sampling the currents stimulated by pulses to the plateaus of the two waves. We have been interested in approaches that can make pulse voltammetry more generally useful for poised systems. In this paper, we present and evaluate two approaches. One, featuring a polarizable reference electrode, proved useful. Another scheme, which depended upon electronic interruption of current at the working electrode, did not succeed in an analytically useful manner. Even so, we describe our experience with it because (a) the reasons for its failure are not obvious, (b) our results bear on reports in the literature, and (c) we include some theory that accounts for the failure and also applies directly to certain cases of normal pulse voltammetry at stationary electrodes.
EXPERIMENTAL Apparatus. All experiments were controlled and monitored via an interface to a Data General Nova 820 minicomputer. The potential program was generated by the computer, which acted through a 12-bit D/A converter driving an auxiliary input to a Princeton Applied Research Model 174 potentiostat. The current response of the electrochemical cell was monitored a t the potentiostat current-to-voltage ( I I E )converter (Princeton Applied Research Model 176) by means of an 8-bit A/D converter (Date1 Model ADC-EH1,4-fis conversion time). Sampled current voltammograms were displayed immediately after acquisition on an oscilloscope driven by parallel 8-bit D/A converters. Individual data sets could also be stored on magnetic tape cassettes for later reference. ANALYTICAL CHEMISTRY, VQL. 49, NO. 3, MARCH 1977
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